1. Technical Field
The invention relates to a thermal release element for sprinklers, valves or the like in the form of a completely enclosed inner space with a container filled with a release liquid.
2. Background Information
The designing of thermal release elements for sprinklers but also for other valves, e.g., for emergency discharge valves of gas containers in the form of containers comprising in their interior a completely enclosed hollow space filled with a release liquid has long been known. These containers are usually formed from glass and can also be designated as small glass casks.
Such small glass casks as thermal release element are described, for example, in DE 36 01 203 A1.
Such release elements are filled with a release liquid that expands upon being heated and result in the bursting of the container typically fixed in a valve seat and holding the valve in closed position so that a release of the valve or the like takes place.
Various substances have been suggested as release liquids. Thus, U.S. Pat. No. 4,938,294 cites toluene, xylene, trichloroethylene, tetrachloroethylene and mixtures of them as suitable release liquids. EP 0 838 242 B1 cites a halogen derivative of an aromatic hydrocarbon with two or more halogen substituents, an aliphatic amide as well as mixtures of them as suitable release liquids.
Finally, DE 197 80 041 C1 describes a substance derived from a halogen-free or halogenated hydrocarbon as a suitable release liquid in which in its structural formula
a)
                i) at least one CH2 group is replaced by oxygen (O), sulfur (S), sulfinyl (SO) or sulfonyl (SO2), or        ii) at least one CH group is replaced by nitrogen,b) no hydrogen atoms directly bonded to oxygen, nitrogen or sulfur occur,c)        i) at least one ring occurs, or        ii) at least two oxygen atoms, each with two single bonds, occur, or        iii) at least two carbonyl groups of ketones and/or aldehydes occur, or        iv) at least one oxidized sulfur atom (SO or SO2) occurs, or        v) at least one nitrogen atom in the form of an amide, imide, imine or nitrile is present.        
All the cited release liquids have proven themselves in principle and are basically suitable for being used in generic release elements. However, there is a continuing need for improvements, in particular under the following viewpoints:
Thus, the requirements for modern release elements have risen as regards the precision of the release temperature, and there is therefore a need for highly temperature-sensitive release liquids that are suitable for an exact and reproducible adjustment of the release temperature in the range of a few degrees Celsius. In addition, many of the previously used release liquids are damaging to health, to the environment or even combustible.